The present invention relates generally to methods and apparatus for using, controlling and detecting radiant energy, and more particularly to light wavelength determination. It is anticipated that the present invention will be employed widely in the calibration of light wavelength and the stabilization of light frequency, particularly in the telecommunications industry, but the invention is also well suited for use in laboratory measurement and in many other industries.
The ability to measure the wavelength or frequency (herein treated as equivalent with both meant when either term is used) of light is highly useful in industry and basic research. The telecommunications industry provides an excellent example, and will be used as the one herein. One brief discussion of the need for this capability in telecommunications can be found in REIZEMAN, xe2x80x9cOptical Nets Brace for Even Heavier Traffic,xe2x80x9d IEEE Spectrum, January 2001, pg. 44-46, discussing the growth of wavelength-division multiplexing (WDM) for communications. This article explains how WDM has grown to 160 wavelength systems today and opines that 320 wavelength systems will be available within one year. It also covers the difficulty of combining and separating light wavelengths, and tunable laser systems are identified as a critical need to make such systems economical and reliable.
Numerous systems exist to measure wavelength or frequency in some manner, these suffer from a number of limitations. Some permit only relative measurement, requiring reliance on a reference standard rather than directly on a principle of physics, and usually also requiring reliance on the system to stay in calibration for some period of time after the reference is removed. Other systems have limited measurement resolution. Still others have complex principles of operation: requiring moving or complex parts, which typically are expensive; or requiring multiple passes through at least part of the optics. In fact, most prior art systems suffer from combinations of these, and summaries of some such systems follow.
U.S. Pat. No. 5,233,405 by Wildnauer et al. teaches a double pass scanning monochromator for use in optical spectrum analysis. It employs a diffraction grating and slit, a motor for rotating the diffraction grating, and a shaft angle encoder for sensing the grating position. As such, the scanning monochromator is a complex apparatus including moving parts for the analysis of a full light spectrum.
U.S. Pat. No. 5,748,310 by Fujiyoshi teaches a spectrum separation apparatus able to generate an output beam having a specific wavelength from a multi-wavelength input beam. An input beam is focused on a diffraction grating to generate a number of diffracted component beams, of which one having a specific wavelength is directed to an output slit. As such, this reference merely addresses spectrum separation, albeit using a diffraction grating and other optical components to achieve this, but it is not otherwise particularly relevant.
U.S. Pat. No. 5,331,651 by Becker et al. teaches a wavelength adjusting system in which a selective filter element, such as a Fabry-Perot etalon, is arranged on a shaft and can be rotated by a motor. When the shaft is rotating, the angle of incidence of a light beam on the filter changes, resulting in a change of the wavelength of the transmitted beam and small changes of the angle of incidence and thus of the wavelength can be adjusted. As such, this system also employs physically moving sub-systems which are difficult to set up, calibrate, and maintain.
U.S. Pat. No. 5,509,023 by Glance et al. teaches a laser tuning system employing a Fabry-Perot resonator and an optical frequency routing device and photodetector system to detect a particular Fabry-Perot resonant frequency to which the laser is tuned. The optical frequency routing device includes a plurality of unequal length input and output waveguides and an optical grating.
U.S. Pat. No. 6,094,271 by Maeda teaches a wavelength measuring system which includes two wavelength dispersion elements (diffraction gratings or prisms) and a right-angle reflecting prism which divides the parallel light rays from the second wavelength dispersion element into two reflected light beams. Two optical receivers then receive the reflected light beams from the right-angle reflecting prism, respectively, and signals from the first and second optical receivers are processed to determine wavelength. As such, this approach uses two gratings or prisms to spectrally disperse the light, the two-reflective surfaces of the right-angle reflecting prism, and two sensors. The gratings or prisms require alignment there between, as well as alignment in relation to the right-angle reflecting prism.
U.S. Pat. No. 5,796,479 by Derickson et al. teaches a detector array spectrometer which simultaneously monitors wavelength, power, and signal-to-noise ratio of wavelength division multiplexed (WDM) channels. A spectrometer formed by a diffraction grating, mirror, and waveplate spatially separates signals from the channels according to wavelengths. The separated signals are then directed incident on an array of split-detectors and noise detectors. As such, this double-pass apparatus directs the light beam onto the diffraction grating, through the waveplate, reflects it off the mirror, back through the waveplate, again onto the diffraction grating, and then onward into splitter and detection components.
U.S. Pat. No. 5,898,502 by Horiuchi et al. teaches an optical wavelength monitoring apparatus. An optical filter is used which maximizes transmittance at a specific wavelength. The transmittance is detected by a photodiode and logarithmic-amplified. The optical filter 24 is an optical element whose transmittance decreases as the incident light deviates from a specific wavelength. As such, this is an enviably simple system. Unfortunately, however, the transmittance of its optical filter is too wavelength specific. That is to say that it is useful for locking to a specific frequency but not so useful for measuring what wavelength is present. It can determine the nature of frequency drift, upward or downward in frequency, but by use of an assembly of plural unequal length waveguides and photodiodes. The amplification and other processing then is substantial.
U.S. Pat. No. 6,061,129 by Ershov et al. teaches a grating spectrometer. A collimated beam is expanded with a (prism) beam expander before illuminating a reflecting grating and then contracted in a second pass through the beam expander before being directed onto a photodiode array. As such, this is also a double-pass apparatus. The placement of the beam expander before the reflecting grating introduces a number of problems. The reflecting grating must accept an expanded beam and thus must be physically larger, and accordingly more expensive. The alignment of the beam expander and the reflecting grating is also critical. Since the light beam must pass twice through the beam expander, any imperfections in and particularly any dust or film that may accumulate on the beam expander thus has two opportunities to effect measurement accuracy.
In sum, the state of the art systems are too complex. They are expensive, difficult to use and maintain, and not as accurate as desired. Accordingly, what is needed is an improved wavelength measurement system.
Accordingly, it is an object of the present invention to provide a simpler yet accurate wavelength meter.
Another object of the invention is to provide a wavelength meter which employs no moving components and, generally, fewer components overall.
And another object of the invention is to provide a wavelength meter which lends itself to use in more complex applications, such as concurrently metering multiple wavelengths or frequency locking in wavelength multiplexed systems.
Briefly, one preferred embodiment of the present invention is a system for measuring the wavelength of light present in a light beam. A light diverter is provided that is able to receive a first beam portion and provide from it a second beam portion having a transverse displacement characteristic. The light beam makes only a single-pass with respect to the light diverter. A light detection unit is further provided that is able to receive the second beam portion, detect the transverse displacement characteristic in it, and provide a raw signal based on this. The raw signal includes at least one intensity value based on the transverse displacement characteristic. A processor is also provided that is able to receive the raw signal and determine the wavelength of light present in the light beam from it.
An advantage of the present invention is that it is simpler and economical. Due to its use of fewer components in a single pass arrangement, the wavelength meter is easily set-up and maintained. Its use of fewer components, and particularly its ability to avoid the use of more complex components, such as curved gratings or complex, high-speed processors keeps the cost of the wavelength meter low.
Another advantage of the invention is that it may be implemented using either a transmissive or reflective light diverter, and a wide range of suitable light diverters may be chosen from to facilitate different goals. For example, transmissive or reflective diffraction gratings or multiple slit plates may be used. One diffraction grating can be used where it is desired to work with multiple channels concurrently. Or a multiple slit plate, optionally with simple a light diverger, can provide an economical arrangement.
Another advantage of the invention is that it may be implemented with a light diverger to enhance resolution, and the range of potential light divergers is broad and may therefore facilitate different goals. The light diverger may also be either transmissive or reflective, particularly facilitating constructing the wavelength meter in a variety of manners. In some embodiments overall compactness can be emphasized and in others susceptibility to undesirable influences can be minimized, like structural change due to temperature variation.
Another advantage of the invention is that it may be implemented with sophisticated signal processing techniques, such as phase sensitive detection, to further increase measurement accuracy and reliability.
And another advantage of the invention is that the problems it solves are pressing and there is a current need in at least one increasingly critical industry, telecommunications, for such a solution.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illustrated in the several figures of the drawings.